This invention relates to a process for producing a phosphor which is used for, for example, a luminous section of a fluorescent display device or the like and adapted to carry out emission of a blue luminous color due to excitation by an electron beam, and more particularly to a process for producing an oxide phosphor free of sulfur (S).
Such a conventional phosphor producing process is disclosed in, for example, Japanese Patent Application Laid-Open Publication No. 168789/1989 and typically practiced in such a way as shown in FIG. 4. More particularly, ZnO and Ga.sub.2 O.sub.3 are fist mixed in equivalent molarities and subject to primary calcination. The primary calcination takes place in an air atmosphere at a temperature of 1300.degree. C. for three hours, so that a ZnO.multidot.Ga.sub.2 O.sub.3 solid solution (mixed crystal) which constitutes a matrix of the phosphor may be prepared. Subsequently, the solid solution is pulverized and Li.sub.3 PO.sub.4 is added thereto in an amount as small as 5.times.10.sup.-2 to 4.times.10.sup.-1 mol per 1 mol of the ZnO.multidot.Ga.sub.2 O.sub.3 solid solution to prepare a mixture, which is then subject to secondary calcination. The secondary calcination takes place in a reducing atmosphere of H.sub.2 +N.sub.2 at a temperature of 1000.degree. C. for one hour, resulting in preparing a ZnO.multidot.Ga.sub.2 O.sub.3 :Li, P phosphor. Then, it is washed with an acid to remove unreacted Li.sub.3 PO.sub.4, leading to purification of the phosphor.
Unfortunately, the conventional phosphor producing process the following disadvantages.
Li.sub.3 PO .sub.4 serves as a flux as well as a doping material, to thereby enhance the melting properties of the matrix. In the prior art, Li.sub.3 PO.sub.4 is added to the matrix not in the primary calcination step but in the secondary calcination step. Thus, it fails to act as a flux during the preparation of the solid solution for the matrix, so that it is required to carry out the primary calcination at a temperature as high as 1300.degree. C., resulting in an increase in heating costs. Also, the primary calcination fails to change all the starting materials to the solid solution irrespective of taking place at a temperature as high as 1300.degree. C., so that the unreacted materials remains in the phosphor product. Further, the pulverization of the calcined solid solution requires much time because it exhibits large bond strength and damages the crystals of the solid solution.
Also, in the prior art, the unreacted or undoped Li.sub.3 PO.sub.4 remaining in the phosphor after the activation of the phosphor in the secondary calcination is removed by washing with nitric acid. Unfortunately, during the washing, nitric acid partially attacks the surface of crystals of the activated phosphor, to thereby deteriorate the surface conditions of the phosphor.
As known in the art, it is the surface section of a phosphor of a depth as small as tens .ANG. that emits light by excitation due to impingement of a low-velocity electron beam thereon in a fluorescent display device or the like. Thus, the surface conditions of a crystalline phosphor substantially affect the luminescence of the phosphor. Accordingly, the above-described disadvantages of the prior art cause the luminous characteristics of a phosphor of such type to be varied.
In addition, in the conventional phosphor producing process, as described above, ZnO and Ga.sub.2 O.sub.3 are mixed in equivalent molarities to prepare the ZnO.multidot.Ga.sub.2 O.sub.3 solid solution serving as the matrix, which is then subject to the secondary calcination in the reducing atmosphere at a temperature of 1,100.degree. C. for the purpose of doping the solid solution with Li and P and removing oxygen from the solid solution. However, as a result of experiments by the inventors, it was found that the secondary calcination causes ZnO in the matrix or solid solution to be reduced to Zn by heating, which is then scattered by vaporization because its boiling point is 930.degree. C. TABLE 1 shows results of the experiments, wherein the residual ratio of each of the ZnO.multidot.Ga.sub.2 O.sub.3 solid solution and the components of the phosphor is indicated by weight percentage.
TABLE 1 ______________________________________ Calcination Temp. (.degree.C.) ZnGa.sub.2 O.sub.4 Atmosphere Time (min) ZnO Ga.sub.2 O.sub.3 Air 1300.degree. C. ______________________________________ 1000 38.3 100 91.1 10 H.sub.2 /N.sub.2 40/160 1000 12.9 100 87.8 (ml/min) 30 1000 0.25 100 80.6 75 ______________________________________
The results shown in TABLE 1 indicate that the secondary calcination causes the amount of Zn contained in the matrix or solid solution to be substantially reduced. Thus, it will be noted that the conventional process exhibits a further disadvantage that the composition of the matrix after the secondary calcination is varied as compared with that prior to the calcination, resulting in luminance of the phosphor being reduced.